The invention pertains to improvements in molding devices of the type utilizing a one-piece and generally hollow, distensible and flexible mold which has an interior configuration corresponding to that of the finished product, and utilizing a vacuum to draw the flexible mold away from the molded article to thereby permit the article to be removed notwithstanding undercut portions formed in the article.
Molding devices employing such a flexible mold and vacuum means are shown in the U.S. Pat. No. 3,776,683 issued Dec. 4, 1973 to Putzer et al. The apparatus disclosed therein comprises a generally hollow one-piece, distensible, flexible mold having a side wall of sufficient thickness whereby it is self-supporting and supports the weight of the material poured therein without distortion of the mold. The flexible mold has a thick and self-supporting radially extending annular flange around the upper portion of the mold such that when the mold is placed within a vacuum pot, the flange supports the mold within the pot and forms a seal around the periphery of the upper edge of the pot. The arrangement is such that the pot and the mold form an annular chamber from which vacuum is drawn to thereby distend and expand the mold into the chamber and away from the formed article because the fluid pressure differential created by the vacuum acts on the flexible mold to expand it, consequently causing its separation from the finished article and thereby permitting the article to be removed from the mold, notwithstanding the backdraft portions. At the same time, the flange cooperates with the edge of the pot to maintain the seal as the mold is pulled away from the article formed. As the mold walls are retracted by the vacuum from the article the mold is generally drawn downwardly somewhat into the pot causing the mold flange to stretch and to be pulled down into the pot to some degree.
In the apparatus disclosed in said U.S. Pat. No. 3,776,683 the shape of the flange does not always provide sufficient means to facilitate accurate centering of the mold and the flange is sometimes drawn into the vacuum pot in the event that the mold and flange are inaccurately centered on the upper edge of the pot. The flange shown in U.S. Pat. No. 3,776,683 has a configuration such that the mold may be easily placed in the pot such that the flange does not overlap the wall of the pot evenly at all points around the circumference. If the mold is inaccurately positioned within the pot it has been found that when vacuum is applied to the pot, that portion of the flange having the least overlap over the edge of the pot wall may be completely pulled into the pot thus breaking the vacuum seal between the upper edge of the pot wall and the lower surface of the flange and thereby causing a failure of the operation of the apparatus.